Specifications for the next generation radio access network (RAN) are currently being developed by the 3rd Generation Partnership Project (3GPP). This so-called next generation RAN is referred to as the “Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”. Another name for E-UTRAN is the “Long Term Evolution (LTE)” RAN.
The LTE RAN is designed to be connected to a core network (CN), which is called the “Evolved Packet Core (EPC) network” or “System Architecture Evolution (SAE) network”. The combination of an E-UTRAN and an EPC network is referred to as an “Evolved Packet System (EPS)” or an “LTE/SAE network”. A base station in an E-UTRAN is called an “E-UTRAN NodeB” (“eNodeB” or “eNB” for short).
There is contemplation of having a base station that provides a coverage area smaller than the coverage area provided by a conventional (i.e. “macro”) cellular base station. For example, it is contemplated to have base station that provides a coverage area for a home only or a campus only. Such a base station is referred to herein as a “home base station.” Other names for a home base station are “Home E-UTRAN NodeB (HeNB)”, “LTE Home Access Point (LTE HAP)”, “LTE Femto Access Point (LTE FAP)” and “femto base station”. In an UTRAN (“Universial Terrestrial Radio Access Network”, also known as 3G) system, a home base station is referred to as a “Home NodeB (HNB)”. For simplicity, we shall refer to a “home base station” as an HeNB.
The HeNB is specified to provide essentially the same service to end users as an eNB and would be connected to the core network by, for example, using some kind of Internet Protocol (IP) based transmission. In this document, the coverage area serviced by an HeNB is called a femtocell (it is also sometimes referred to as a picocell when, for example, the coverage area provided by the HeNB encompasses a large office building or campus), and the coverage area serviced by an eNB is called a macrocell. Thus, an HeNB (or other home base station) may be referred to herein as a “femto base station” and an eNB (or other base station that provides a larger coverage area than a femto base station) may be referred to as a “macro base station”.
A perceived advantage of an HeNB is that it may be cheaper for an end user to receive a service when using an HeNB for network access versus using an eNB for network access. The HeNB would, in most cases, use the end user's already existing broadband connection (e.g. xDSL, Cable, etc.) to achieve connectivity to an operator's CN and possibly to other eNBs/HeNBs.
The current working assumption in the 3GPP LTE RAN specification is that the “X2 interface” is not used with HeNBs, but the X2 interface is used by eNBs to communicate with each other. The X2 interface between eNBs is used for handover, so called ‘X2 based handover’ and for Inter-cell Interference Control (ICIC). When an X2 connection is set up between two eNBs, the eNBs exchange information regarding the macrocells served by the eNBs. Typically, an X2 connection is set up only between eNBs that serve cells between which handover may be performed. The protocol used for the X2 control plane is called the “X2 Application Protocol (X2AP)”. The X2AP messages used for establishing an X2 connection between two eNBs are X2 SETUP REQUEST and X2 SETUP RESPONSE.
A mobile network may have up to a million or more HeNBs. It is doubtful that the capacity of the control nodes in the CN (e.g. a Mobility Management Entity (MME) or other core network control nodes) will be able to handle that many HeNBs. Therefore, a gateway (a.k.a. a HeNB gateway (HeNB-GW), which is a type of concentrator node) is used to conceal the large number of HeNBs from the CN. That is, the HeNB-GW will, from the perspective of the CN, look like an eNB with many cells. Accordingly, the HeNB-GW communicates with the control nodes of the CN using the “S1 interface” in the same manner that an eNB communicates with the control nodes of the CN using the S1 interface. Functionally, the HeNB-GW acts as a proxy of the CN control node for all the HeNBs that are connected to the HeNB-GW (i.e. from the perspective of an HeNB, the HeNB-GW will look like a CN control node). That is, an HeNB communicates with a HeNB-GW using an S1 interface in the same manner that an eNB communicates with the control nodes of the CN using the S1 interface. In an UTRAN system, the gateway is referred to as a Home NodeB Gateway or HNB-GW.
The number of HeNBs connected to an HeNB-GW may be up to 100,000 or more. Thus, the number of femtocells “served” by an HeNB-GW may be up to 200,000 or more, assuming an HeNB serves up to two femtocells.
There exist two different procedures for performing a handover between eNBs. These are: (1) the “S1 based” handover procedure and (2) the “X2 based” handover procedure. As these names imply, the S1 based handover procedure uses an S1 interface between nodes of the E-UTRAN and the control nodes in the CN, whereas the X2 based handover procedure uses mainly the X2 interface between nodes of the E-UTRAN. The S1 based handover procedure is described in 3GPP TS 23.401, and the X2 based handover procedure is described in 3GPP TS 36.300. Both the S1 based handover procedure and the X2 based handover procedure can be applied to handovers between HeNBs. However, the latter only in case the X2 interface is used.
A problem with using the S1 based handover procedures to transfer a connection from a source HeNB to a target HeNB is that the S1 based handover procedure requires signaling towards a core network control node. Because it is expected that the RAN will include a large number of HeNBs, it is expected that there will be a large number of handovers between HeNBs in, for example, an enterprise or campus scenario. Consequently, using the S1 based handover procedure in the femto environment may significantly increase the signaling load seen by the core network control nodes. The X2 based handover procedure can decrease this signaling load, but this would require that the HeNBs implement at least some aspects of an X2 protocol, which would increase the cost and complexity of the HeNBs. Because HeNBs are expected to be mass-market devices, it is desired to keep HeNBs as simple and cheap as possible.
What is desired is an improved handover procedure.